Large passenger aircraft are typically struck by lightning once or twice a year, each lightning bolt striking with up to 200,000 amps of electrical current that seeks the path of least electrical resistance. Many modern passenger aircraft have exterior surfaces made from composite materials which have a very high electrical resistance. There is therefore a high probability of lightening attachment at any of the many metallic fasteners in the exterior surface, which have a much lower electrical resistance. In the wing, some of these fasteners pass through the outer wing skin into the fuel tank.
FIG. 1 is a side view of part of a fastener assembly passing through a panel 1, which may be a composite or metallic panel. The assembly comprises a fastener comprising an externally threaded bolt 2, an internally threaded nut 3, and a washer 4 (the fastener may alternatively comprise any other known fastener type, such as a rivet or swage fastener). In the event of a lightning strike hitting the panel 1 and attaching to the fastener, sparking, plasma or out-gassing may occur at the locations indicated by reference 5 in FIG. 1. The panel 1 may provide a fuel tank boundary and the fastener may therefore be immersed in fuel or fuel vapour rich gas. A lightning strike at the fastener may thus provide sparking and hot gas ignition sources which could cause ignition of the fuel.
A known method of providing spark suppression is described in EP-A-0334011. A volume of gas is enclosed by a cap around the fastener. The gas provides spark suppression for arcing that may occur between the composite structure and the metal fastener during any lightning strike.